1. Field of the Invention
This invention relates to a light receiving device for use in devices for monitoring an output of semiconductor laser, receiver devices used in optical communication systems, etc.
2. Related Background Art
FIG. 1A shows a top view of a conventional light receiving device, and FIG. 1B shows a sectional view along the line X-X' thereof.
A buffer layer 2a, a light absorbing layer 2b, and a window layer 2c are sequentially formed, in this stated order as a first conduction-type lightly doped semiconductor layer 2 on a surface of a first conduction-type highly doped semiconductor substrate 1 having a first conduction-type electrode 6 on the backside. At an element forming region (a region where a PIN photo-diode structure to be formed) which is a part of the first conduction-type semiconductor layer 2, there is formed a second conduction-type region 3 on which impurities are selectively diffused. An anti-reflection coating 4, a second conduction-type electrode 5, and a device protection film (i.e. passivation film) 7 are formed on a surface of the second conduction-type region 3.
When a reverse bias is applied to the light receiving device of the above-described structure, a depletion layer is expanded in a pn junction portion of the first conduction-type semiconductor layer 2. In this state, when light pulses or the like are incident on a neighborhood of the pn junction portion through the window layer 2c, carriers are generated in this light detecting region 8. The generated carriers are separated and accelerated by an electric field in this depletion layer to be output as a photoelectric current through the electrodes 5 and 6.
In the light receiving device of FIGS. 1A and 1B, carriers generated outside the light detecting region 8 are concurrently detected. In other words, even these carriers are output as a photoelectric current because carriers generated by light pulses entering outside the element forming region are diffused in the device forming region due to a density gradient. In this case, the transfer of the carriers due to the diffusion is so slow that the response waveforms of the light pulses have a tail at the end of waveform as shown on curve B in FIG. 2.
It is possible to solve this problem by the perfect control of the incidence position of light pulses or others. Practically, however, it is difficult to cause light pulses to be incident on the inside of the element forming region. Especially in optical communication applications for detecting optical signals from optical fibers, etc., the area of the element forming region, i.e., the light detecting region 8, is made small for the purpose of the high-speed response of the light detecting device. As a result, the current component attributed to the diffusion of carriers generated outside the light detecting region 8 is increased, and the response speed of the light detecting device is reduced. This problem occurs similarly with the light receiving device used in controlling drive-currents for semiconductor lasers.